The invention relates to a method for influencing a pre-drained fibrous web on a screen, in which the fibrous web temporarily undergoes a surface compression by a second screen, and an apparatus for performing such a method.
In conventional methods of this type, a surface compression is exerted on a pre-drained fibrous web lying on a longitudinal screen by means of a dandy roll consisting of either a screen roller alone or a screen roller disposed within a closed, circulating screen. This surface compression either applies a watermark to the fibrous web or reorients the uppermost fiber layer to achieve a greater uniformity. The disadvantage of these methods is that the zone in which the surface compression takes place cannot be freely varied; the restrictions thereon being determined, first of all by the operational speed of the machine and secondly, by the diameter of the dandy roll. An additional disadvantage of the known method is that the setting of the reoriented fibers takes place solely by the screen roller or the screen rotating together therewith separating from the fibrous web. By this means, however, the desired reorientation is partially reversed or cancelled. Furthermore, especially at high machine speeds, difficulties arise in that a portion of the water initially remains in the fabric of the dandy roll or the screen and is then thrown off in the form of droplets. These droplets then strike the newly reoriented fibrous web and again change the attained fiber positions.